Surgical access system and related methods

ABSTRACT

A system for accessing a surgical target site and related methods, involving an initial distraction system for creating an initial distraction corridor, and an assembly capable of distracting from the initial distraction corridor to a secondary distraction corridor and thereafter sequentially receiving a plurality of retractor blades for retracting from the secondary distraction corridor to thereby create an operative corridor to the surgical target site, both of which may be equipped with one or more electrodes for use in detecting the existence of (and optionally the distance and/or direction to) neural structures before, during, and after the establishment of an operative corridor to a surgical target site.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/682,568, filed Oct. 8, 2003, which claims the benefit of priorityfrom U.S. Provisional Patent Application Ser. No. 60/417,235, filed Oct.8, 2002, the entire contents of these applications are hereby expresslyincorporated by reference into this disclosure as if set forth fullyherein. The present application also incorporates by reference thefollowing patent applications in their entireties (collectively, the“NeuroVision Applications”): PCT App. Ser. No. PCT/US02/22247, entitled“System and Methods for Determining Nerve Proximity, Direction, andPathology During Surgery,” filed on Jul. 11, 2002; PCT App. Ser. No.PCT/US02/30617, entitled “System and Methods for Performing SurgicalProcedures and Assessments,” filed on Sep. 25, 2002; PCT App. Ser. No.PCT/US02/35047, entitled “System and Methods for Performing PercutaneousPedicle Integrity Assessments,” filed on Oct. 30, 2002; PCT App. Ser.No. PCT/US03/02056, entitled “System and Methods for Determining NerveDirection to a Surgical Instrument,” filed Jan. 15, 2003.

BACKGROUND

I. Field

The present invention relates generally to systems and methods forperforming surgical procedures and, more particularly, for accessing asurgical target site in order to perform surgical procedures.

II. Description of Related Art

A noteworthy trend in the medical community is the move away fromperforming surgery via traditional “open” techniques in favor ofminimally invasive or minimal access techniques. Open surgicaltechniques are generally undesirable in that they typically requirelarge incisions and high amounts of tissue displacement to gain accessto the surgical target site, which produces concomitantly high amountsof pain, lengthened hospitalization (increasing health care costs), andhigh morbidity in the patient population. Less-invasive surgicaltechniques (including so-called “minimal access” and “minimallyinvasive” techniques) are gaining favor due to the fact that theyinvolve accessing the surgical target site via incisions ofsubstantially smaller size with greatly reduced tissue displacementrequirements. This, in turn, reduces the pain, morbidity and costassociated with such procedures. The access systems developed to date,however, fail in various respects to meet all the needs of the surgeonpopulation.

One drawback associated with prior art surgical access systems relatesto the ease with which the operative corridor can be created, as well asmaintained over time, depending upon the particular surgical targetsite. For example, when accessing surgical target sites located beneathor behind musculature or other relatively strong tissue (such as, by wayof example only, the psoas muscle adjacent to the spine), it has beenfound that advancing an operative corridor-establishing instrumentdirectly through such tissues can be challenging and/or lead to unwantedor undesirable effects (such as stressing or tearing the tissues). Whilecertain efforts have been undertaken to reduce the trauma to tissuewhile creating an operative corridor, such as (by way of example only)the sequential dilation system of U.S. Pat. No. 5,792,044 to Foley etal., these attempts are nonetheless limited in their applicability basedon the relatively narrow operative corridor. More specifically, based onthe generally cylindrical nature of the so-called “working cannula,” thedegree to which instruments can be manipulated and/or angled within thecannula can be generally limited or restrictive, particularly if thesurgical target site is a relatively deep within the patient.

Efforts have been undertaken to overcome this drawback, such as shown inU.S. Pat. No. 6,524,320 to DiPoto, wherein an expandable portion isprovided at the distal end of a cannula for creating a region ofincreased cross-sectional area adjacent to the surgical target site.While this system may provide for improved instrument manipulationrelative to sequential dilation access systems (at least at deep siteswithin the patient), it is nonetheless flawed in that the deployment ofthe expandable portion may inadvertently compress or impinge uponsensitive tissues adjacent to the surgical target site. For example, inanatomical regions having neural and/or vasculature structures, such ablind expansion may cause the expandable portion to impinge upon thesesensitive tissues and cause neural and/or vasculature compromise, damageand/or pain for the patient.

This highlights yet another drawback with the prior art surgical accesssystems, namely, the challenges in establishing an operative corridorthrough or near tissue having major neural structures which, ifcontacted or impinged, may result in neural impairment for the patient.Due to the threat of contacting such neural structures, efforts thus farhave largely restricted to establishing operative corridors throughtissue having little or substantially reduced neural structures, whicheffectively limits the number of ways a given surgical target site canbe accessed. This can be seen, by way of example only, in the spinalarts, where the exiting nerve roots and neural plexus structures in thepsoas muscle have rendered a lateral or far lateral access path(so-called trans-psoas approach) to the lumbar spine virtuallyimpossible. Instead, spine surgeons are largely restricted to accessingthe spine from the posterior (to perform, among other procedures,posterior lumbar interbody fusion (PLIF)) or from the anterior (toperform, among other procedures, anterior lumbar interbody fusion(ALIF)).

Posterior-access procedures involve traversing a shorter distance withinthe patient to establish the operative corridor, albeit at the price ofoftentimes having to reduce or cut away part of the posterior bonystructures (i.e. lamina, facets, spinous process) in order to reach thetarget site (which typically comprises the disc space). Anterior-accessprocedures are relatively simple for surgeons in that they do notinvolve reducing or cutting away bony structures to reach the surgicaltarget site. However, they are nonetheless disadvantageous in that theyrequire traversing through a much greater distance within the patient toestablish the operative corridor, oftentimes requiring an additionalsurgeon to assist with moving the various internal organs out of the wayto create the operative corridor.

The present invention is directed at eliminating, or at least minimizingthe effects of, the above-identified drawbacks in the prior art.

SUMMARY

The present invention accomplishes this goal by providing a novel accesssystem and related methods which involve: (1) distracting the tissuebetween the patient's skin and the surgical target site to create anarea of distraction (otherwise referred to herein as a “distractioncorridor”); (2) retracting the distraction corridor to establish andmaintain an operative corridor; and/or (3) detecting the existence of(and optionally the distance and/or direction to) neural structuresbefore, during and after the establishment of the operative corridorthrough (or near) any of a variety of tissues having such neuralstructures which, if contacted or impinged, may otherwise result inneural impairment for the patient.

As used herein, “distraction” or “distracting” is defined as the act ofcreating a corridor (extending to a location at or near the surgicaltarget site) having a certain cross-sectional area and shape(“distraction corridor”), and “retraction” or “retracting” is defined asthe act of creating an operative corridor by increasing or maintainingthe cross-sectional area of the distraction corridor (and/or modifyingits shape) with at least one retractor blade such that surgicalinstruments can be passed through operative corridor to the surgicaltarget site. It is expressly noted that, although described hereinlargely in terms of use in spinal surgery, the access system of thepresent invention is suitable for use in any number of additionalsurgical procedures, including those wherein tissue having significantneural structures must be passed through (or near) in order to establishan operative corridor.

According to one broad aspect of the present invention, the accesssystem comprises a tissue distraction assembly and a tissue retractionassembly. The tissue distraction assembly (in conjunction with one ormore elements of the tissue retraction assembly) is capable of, as aninitial step, distracting a region of tissue between the skin of thepatient and the surgical target site. The tissue retraction assembly iscapable of, as a secondary step, being introduced into this distractedregion to thereby define and establish the operative corridor. Onceestablished, any of a variety of surgical instruments, devices, orimplants may be passed through and/or manipulated within the operativecorridor depending upon the given surgical procedure.

The tissue distraction assembly may include any number of componentscapable of performing the necessary distraction. By way of example only,the tissue distraction assembly may include a K-wire, an initial dilatorof split construction, and one or more dilators of traditional (that is,non-split) construction for performing the necessary tissue distractionto receive the remainder of the tissue retractor assembly thereafter.One or more electrodes may be provided on one or more of the K-wire anddilator(s) to detect the presence of (and optionally the distance and/ordirection to) neural structures during tissue distraction.

The tissue retraction assembly may include any number of componentscapable of performing the necessary retraction. By way of example only,the tissue retraction assembly may include one or more retractor bladesextending proximally from the surgical target site for connection with aportion (more specifically, a pivot linkage assembly) of the speculumassembly. The retractor blades may be equipped with a mechanism fortransporting or emitting light at or near the surgical target site toaid the surgeon's ability to visualize the surgical target site,instruments and/or implants during the given surgical procedure.According to one embodiment, this mechanism may comprise, but need notbe limited to, providing one or more strands of fiber optic cable withinthe walls of the retractor blades such that the terminal (distal) endsare capable of emitting light at or near the surgical target site.According to another embodiment, this mechanism may comprise, but neednot be limited to, constructing the retractor blades of suitablematerial (such as clear polycarbonate) and configuration such that lightmay be transmitted generally distally through the walls of the retractorblade light to shine light at or near the surgical target site. This maybe performed by providing the retractor blades having light-transmissioncharacteristics (such as with clear polycarbonate construction) andtransmitting the light almost entirely within the walls of the refractorblade (such as by frosting or otherwise rendering opaque portions of theexterior and/or interior) until it exits a portion along the interior(or medially-facing) surface of the retractor blade to shine at or nearthe surgical target site. The exit portion may be optimally configuredsuch that the light is directed towards the approximate center of thesurgical target site and may be provided along the entire innerperiphery of the retractor blade or one or more portions therealong.

The retractor blades may be optionally dimensioned to receive and directa rigid shim element to augment the structural stability of theretractor blades and thereby ensure the operative corridor, onceestablished, will not decrease or become more restricted, such as mayresult if distal ends of the retractor blades were permitted to “slide”or otherwise move in response to the force exerted by the displacedtissue. In a preferred embodiment, only the posterior and anteriorretractor blades are equipped with such rigid shim elements, which areadvanced into the disc space after the posterior and anterior retractorblades are positioned. The rigid shim elements are preferably orientedwithin the disc space such that they distract the adjacent vertebralbodies, which serves to restore disc height. They are also preferablyadvanced a sufficient distance within the disc space (preferably pastthe midline), which serves the dual purpose of preventing post-operativescoliosis and forming a protective barrier (preventing the migration oftissue (such as nerve roots) into the operative field and theinadvertent advancement of instruments outside the operative field).

According to yet another aspect of the present invention, any number ofdistraction assemblies and/or retraction assemblies (including but notlimited to those described herein) may be equipped to detect thepresence of (and optionally the distance and/or direction to) neuralstructures during the steps tissue distraction and/or retraction. Toaccomplish this, one or more stimulation electrodes are provided on thevarious components of the distraction assemblies and/or retractionassemblies, a stimulation source (e.g. voltage or current) is coupled tothe stimulation electrodes, a stimulation signal is emitted from thestimulation electrodes as the various components are advanced towardsthe surgical target site, and the patient is monitored to determine ifthe stimulation signal causes muscles associated with nerves or neuralstructures within the tissue to innervate. If the nerves innervate, thisindicates that neural structures may be in close proximity to thedistraction and/or retraction assemblies.

This monitoring may be accomplished via any number of suitable fashions,including but not limited to observing visual twitches in muscle groupsassociated with the neural structures likely to found in the tissue, aswell as any number of monitoring systems. In either situation(traditional EMG or surgeon-driven EMG monitoring), the access system ofthe present invention may advantageously be used to traverse tissue thatwould ordinarily be deemed unsafe or undesirable, thereby broadening thenumber of manners in which a given surgical target site may be accessed.

BRIEF DESCRIPTION OF THE DRAWINGS

Many advantages of the present invention will be apparent to thoseskilled in the art with a reading of this specification in conjunctionwith the attached drawings, wherein like reference numerals are appliedto like elements and wherein:

FIG. 1 is a perspective view of a tissue retraction assembly (in use)forming part of a surgical access system according to the presentinvention, including a posterior retractor blade 12, an anteriorretractor blade 14, and two supplemental retractor blades 16, 18;

FIGS. 2 and 3 are front and back views, respectively, of the posteriorand anterior retractor blades 12, 14 shown in FIG. 1;

FIGS. 4 and 5 are front and back views, respectively, of shim element22, 24 according to the present invention, dimensioned to be engagedwith the inner surface of the posterior and anterior retractor blades ofFIG. 1 for the purpose of positioning a shim extension 80 within thedisc space;

FIG. 6 is a perspective view illustrating the back of the supplementalretractor blades 16, 18 shown in FIG. 1 according to the presentinvention;

FIG. 7 is a perspective view illustrating the components and use of aninitial distraction assembly 40 (i.e. K-wire 42, an initial dilatingcannula 44 with handle 46, and a split-dilator 48 housed within theinitial dilating cannula 44) forming part of the surgical access systemaccording to the present invention, for use in distracting to a surgicaltarget site (e.g. disk space);

FIG. 8 is a perspective view illustrating the K-wire 42 andsplit-dilator 48 of the initial distraction assembly 40 with the initialdilating cannula 44 and handle 46 of FIG. 7 removed;

FIG. 9 is a posterior view of the vertebral target site illustrating thesplit-dilator 48 of the present invention in use distracting in agenerally cephalad-caudal fashion according to one aspect of the presentinvention;

FIG. 10 is a perspective view illustrating the split-dilator 48 of thepresent invention in use distracting in a generally posterior-anteriorfashion according to another aspect of the present invention and in usewith a posterior retractor blade 12 forming part of the retractorassembly 10 of the present invention;

FIG. 11 is a perspective view illustrating a shim introducer 51introducing the posterior shim element 22 such that the shim extension80 is positioned within the disc space;

FIG. 12 is a perspective view illustrating an anterior retractor blade14 forming part of the refractor assembly 10 of the present inventionbeing introduced with a gripping assembly 53 in association with theanterior half 48 b of the split dilator 48 of the present invention;

FIG. 13 is a perspective view of the anterior retractor blade 14 beingmoved away from the posterior retractor blade 12 through the use of aplurality of sequentially dilating cannula 50;

FIG. 14 is a perspective view illustrating the introduction ofsupplemental retractor blades 16, 18 according to the present invention;

FIG. 15 is a perspective view illustrating a shim introducer 51introducing the anterior shim element 24 such that the shim extension 80is positioned within the disc space;

FIG. 16 is a perspective view of an exemplary nerve monitoring systemcapable of performing nerve monitoring before, during and after thecreating of an operative corridor to a surgical target site using thesurgical access system in accordance with the present invention;

FIG. 17 is a block diagram of the nerve monitoring system shown in FIG.16; and

FIGS. 18-19 are screen displays illustrating exemplary features andinformation communicated to a user during the use of the nervemonitoring system of FIG. 16.

DETAILED DESCRIPTION

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure. The surgical access system disclosed herein boasts avariety of inventive features and components that warrant patentprotection, both individually and in combination.

It is furthermore to be readily understood that, although discussedbelow primarily within the context of spinal surgery, the surgicalaccess system and related methods of the present invention may findapplicability in any of a variety of surgical and/or medicalapplications such that the following description relative to the spineis not to be limiting of the overall scope of the present invention.Moreover, while described below employing the nerve monitoring featuresdescribed above (otherwise referred to as “nerve surveillance”) duringspinal surgery, it will be appreciated that such nerve surveillance willnot be required in all situations, depending upon the particularsurgical target site (e.g. disk space, vertebral body, and/or internalorgan) and surgical approach (e.g. lateral, posterior, anterior, and/orpostero-lateral approaches to the spine).

The present invention is directed at a novel surgical access system andrelated methods which involve creating and maintaining an operativecorridor to the surgical target site, and optionally detecting theexistence of (and optionally the distance and/or direction to) neuralstructures before, during and/or after this process (including the stepsof distraction and/or retraction).

Distraction followed by retraction is advantageous because it providesthe ability to more easily position an operative corridor-establishingdevice through tissue that is strong, thick or otherwise challenging totraverse in order to access a surgical target site. The variousdistraction systems of the present invention are advantageous in thatthey provide an improved manner of atraumatically establishing adistraction corridor prior to the use of the retraction systems of thepresent invention. The various retractor systems of the presentinvention are advantageous in that they provide an operative corridorhaving improved cross-sectional area and shape (including customizationthereof) relative to the prior art surgical access systems. Moreover, byoptionally equipping the various distraction systems and/or retractionsystems with one or more electrodes, an operative corridor may beestablished through (or near) any of a variety of tissues having suchneural structures which, if contacted or impinged, may otherwise resultin neural impairment for the patient.

The present invention involves accessing a surgical target site in afashion less invasive than traditional “open” surgeries and doing so ina manner that provides access in spite of the neural structures requiredto be passed through (or near) in order to establish an operativecorridor to the surgical target site. Generally speaking, the surgicalaccess system of the present invention accomplishes this by providing atissue distraction assembly and a tissue retraction assembly, both ofwhich may be equipped with one or more electrodes for use in detectingthe existence of (and optionally the distance and/or direction to)neural structures. These electrodes are preferably provided for use witha nerve surveillance system such as, by way of example, the type shownand described in the NeuroVision Applications referenced above, theentire contents of which are expressly incorporated by reference as ifset forth herein in their entirety.

Generally speaking, this nerve surveillance is capable of detecting theexistence of (and optionally the distance and/or direction to) neuralstructures during the distraction and retraction of tissue by detectingthe presence of nerves by applying a stimulation signal to suchinstruments and monitoring the evoked EMG signals from the myotomesassociated with the nerves being passed by the distraction andretraction systems of the present invention. In so doing, the system asa whole (including the surgical access system of the present invention)may be used to form an operative corridor through (or near) any of avariety of tissues having such neural structures, particularly thosewhich, if contacted or impinged, may otherwise result in neuralimpairment for the patient. In this fashion, the access system of thepresent invention may be used to traverse tissue that would ordinarilybe deemed unsafe or undesirable, thereby broadening the number ofmanners in which a given surgical target site may be accessed.

The tissue distraction assembly of the present invention (comprising aK-wire, an initial dilator, and a split-dilator disposed within theinitial dilator) is employed to distract the tissues extending betweenthe skin of the patient and a given surgical target site (preferablyalong the posterior region of the target intervertebral disc) such that,once distracted, the resulting void or distracted region within thepatient is of sufficient size to accommodate the introduction of aposterior retractor blade forming part of a tissue retraction assemblyof the present invention. This forms the posterior border of theresulting operative corridor. Following (or contemporaneous with) this,a posterior shim element (which is preferably slideably engaged with theposterior retractor blade) may be advanced such that a shim extension inpositioned within the posterior region of the disc space. An anteriorretractor blade may then be introduced (in generally abutting relationwith the posterior retractor blade) and thereafter moved anteriorly toincrease the AP (or “width”) dimension of the operative corridor. Oncein the appropriate anterior position, the anterior retractor blade maybe locked in position and, thereafter, an anterior shim element advancedtherealong for positioning a shim extension within the anterior of thedisc space. The shim elements serve to distract the adjacent vertebralbodies (thereby restoring disc height), to form protective barriers(against the migration of tissue into (or instruments out of) theoperative site), and to rigidly couple the posterior and anteriorretractor blades in fixed relation relative to the vertebral bodies.First and second supplemental retractor blades (disposed caudal andcephalad) are also preferably employed to establish and maintain the“height” dimension of the operative corridor. Once established, any of avariety of surgical instruments, devices, or implants may be passedthrough and/or manipulated within the operative corridor depending uponthe given surgical procedure.

FIG. 1 illustrates a tissue retraction assembly 10 forming part of asurgical access system according to the present invention. Theretraction assembly 10 includes a posterior retractor blade 12, ananterior retractor blade 14, and supplemental retractor blades 16, 18,all of which are coupled to a mounting structure 20. Posterior andanterior retractor blades 12, 14 establish an AP (or “width”) dimensionof an operative corridor 15. Posterior retractor blade 12 and anteriorretractor blade 14 are equipped with shim elements 22, 24, respectively.Shim elements 22, 24 serve to distract the adjacent vertebral bodies(thereby restoring disc height), form protective barriers (against themigration of tissue into (or instruments out of) the operative site),and rigidly couple the posterior and anterior retractor blades 12, 14 infixed relation relative to the vertebral bodies. First and secondsupplemental retractor blades 16, 18 (disposed caudal and cephalad)establish and maintain the “height” dimension of the operative corridor15.

Any number of suitable mounting units (not shown) may be employed tomaintain the retraction assembly 10 in a fixed and rigid fashionrelative to the patient. The mounting structure 20 may be coupled to anynumber of mechanisms for rigidly registering the mounting structure 20in fixed relation to the operative site, such as through the use of anarticulating arm mounted to the operating table. The mounting structure20 includes posterior and anterior struts 26, 28 disposed in hingedrelation to fixed strut members 30, 32. The hinged nature of struts 26,28 allows the posterior and anterior refractor blades 12, 14 to beadjusted independently of one another. The proximal portion of each ofthe retractor blades 12-18 is preferably provided in a split or forkedfashion to accommodate locking assemblies 36 for locking the position ofthe refractor blades 12-18 with respect to the mounting assembly 20.

The retractor blades 12, 14 (FIGS. 2-3) are dimensioned to detachablycouple with the shim elements 22, 24 (FIGS. 4-5), respectively. In oneembodiment, shown best in FIG. 3, this is accomplished by providing theinner surface 72 of each retractor blade 12, 14 with an engagementgroove 70 (having, by way of example, a female dove-tail configurationas shown) along the midline thereof and one or more recess 74 disposedat or near the distal end. As best seen in FIG. 5, the exterior surface73 of each shim element 22, 24 is provided with an elongated engagementmember 76 (having, by way of example, a male dove-tail configuration asshown) dimensioned to be slideably received within the engagement groove70 of the retractor blades 12, 14, along with a pair of generallysquare-shaped engagement members 78 dimensioned to “snap” into therecesses 74 disposed at the distal ends of the refractor blades 12, 14.

The inner surface 75 of each shim elements 22, 24 (FIG. 4) is providedwith a generally concave region 77 having an aperture 79 formed thereinfor engagement with a shim introducer (shown generally in FIGS. 11 and15) for the purpose of controlling the engagement between the shimelements 22, 24 and the retractor blades 12, 14, respectively, as wellas the advancement of the distal regions 80 of the shim elements 22, 24into the surgical target site (e.g. disc space). As best seen in FIGS. 2and 5, the retractor blades 12, 14 and shim elements 22, 24 may berespectively provided with one or more electrodes 82, 84 for use inundertaking the nerve surveillance techniques described herein. The sameis true for supplemental retractor blades 16, 18 (FIG. 6), which mayalso be provided with one or more electrodes 86 for use in undertakingthe nerve surveillance techniques described herein.

The retractor blades 12-18, as well as the shim elements 22, 24 mayoptionally be equipped with any number of different mechanisms fortransporting or emitting light at or near the surgical target site toaid the surgeon's ability to visualize the surgical target site,instruments and/or implants during the given surgical procedure. Forexample, one or more strands of fiber optic cable may be coupled tothese components such that light may be delivered from a light sourceand selectively emitted into the operative corridor and/or the surgicaltarget site.

This may be accomplished, by way of example only, by constructing theretractor blades 12-18 and/or shim elements 22, 24 of suitable material(such as clear polycarbonate) and configuration such that light may betransmitted generally distally through a light exit region formed alongthe entire inner periphery thereof and located in the general vicinityas the operative corridor 15. This may be performed by providing therefractor blades 12-18 and/or the shim elements 22, 24 withlight-transmission characteristics (such as with clear polycarbonateconstruction) and transmitting the light almost entirely within thewalls of the refractor blades 12-18 and/or shim elements 22, 24 (such asby frosting or otherwise rendering opaque portions of the exteriorand/or interior and coupling the light source thereto such as via aport) until it exits a portion along the interior surface thereof toshine at or near the surgical target site.

In one embodiment, a variety of sets of retractor blades 12-18 and/orshim elements 22, 24 may be provided, each having a different length toaccount for any number of possible surgical target sites and/oranatomical configurations. In a further embodiment, each set ofrefractor blades 12-18 and/or shim elements 22, 24 may be marked orcolor-coded to aid in indicating to the surgeon the particular length ofthe blade 12-18 and/or shim element 22, 24 (and/or extension 80 of theshim elements 22, 24) and/or the depth of the surgical target site.

The retractor blades 12-18 and shim elements 22, 24 may be constructedfrom any number of materials suitable for medical applications,including but not limited to plastics, metals, ceramics or anycombination thereof. Depending on the construction, some or all of thesedevices may be disposable (i.e. single use) and/or reusable (i.e.multi-use).

FIG. 7 illustrates an initial distraction assembly 40 forming part ofthe surgical access system according to the present invention. Theinitial tissue distraction assembly 40 is employed to perform an initialdistraction of tissue from the skin of the patient down to or near thesurgical target site, prior to the introduction of the tissue retractionassembly 10 shown and described above with reference to FIGS. 1-6. Byway of example, this is accomplished by providing the initialdistraction assembly 40 as including a K-wire 42, an initial dilatingcannula 44 with handle 46, and a split-dilator 48 housed within theinitial dilating cannula 44. The initial tissue distraction assembly 40may be constructed from any number of materials suitable for medicalapplications, including but not limited to plastics, metals, ceramics orany combination thereof. Depending on the construction, some or all ofthe tissue distraction assembly 40 may be disposable (i.e. single use)and/or reusable (i.e. multi-use). As will be discussed below in greaterdetail), the K-wire 42, initial dilating cannula 44, and split-dilator48 may be provided with electrodes 60, 62, 64, respectively, for thepurpose of determining the location of nerves or neural structuresrelative to these components as they are advanced towards or positionedat or near the surgical target site.

The K-wire 42 is preferably constructed having generally narrow diameter(such as, by way of example only, 1.5 mm) and sufficient rigidity andstrength such that it can pierce the skin of the patient and be advancedthrough the intervening tissue to reach the surgical target site. TheK-wire 42 also preferably includes indicia for determining the distancebetween a distal end thereof and the skin of the patient. Thesplit-dilator 48 and dilating cannula 44 are inner and outer dilatingelements, respectively, capable of being simultaneously introduced overthe K-wire 42 for the purpose of further distracting the tissuepreviously distracted by the K-wire 42.

The split-dilator 48 is preferably constructed having an inner diameterapproximating the diameter of the K-wire 42 (such as, by way of exampleonly, 1.5 mm), an outer diameter of increased dimension (such as, by wayof example only, 6.5 mm), and indicia for determining the distancebetween a distal end 52 and the skin of the patient. The initialdilating cannula 44 is similarly preferably constructed having an innerdiameter approximating the outer diameter of the split-dilator 48 (suchas, by way of example only, 6.5 mm), an outer diameter of increaseddimension (such as, by way of example only, 7.5 mm), and indicia fordetermining the distance between a distal end 54 and the skin of thepatient. The respective lengths of the K-wire 42, dilating cannula 44,and split-dilator 48 may vary depending upon the given surgical targetsite (that is, the “depth” of the surgical target site within thepatient). It will be similarly appreciated that the diameters anddimensions for these elements may also vary depending upon theparticular surgical procedure. All such surgically appropriatevariations (length, diameter, etc. . . . ) are contemplated as fallingwithin the scope of the present invention.

In use, the K-wire 42 and split-dilator 48 are disposed within theinitial dilating cannula 44 and the entire assembly 40 advanced throughthe tissue towards the surgical target site (e.g. disk space) as shownin FIG. 7. After the initial dilating assembly 40 is advanced such thatthe distal ends of the split-dilator 48 and initial dilating cannula 44are positioned within the disc space (FIG. 7), the initial dilator 44and handle 46 are removed (FIG. 8) to thereby leave the split-dilator 48and K-wire 42 in place. As shown in FIG. 9, the split-dilator 48 isthereafter split such that the respective halves 48 a, 48 b areseparated from one another to distract tissue in a generallycephalad-caudal fashion relative to the target site. The split dilator48 may thereafter be relaxed (allowing the dilator halves 48 a, 48 b tocome together) and rotated such that the dilator halves 48 a, 48 b aredisposed in the anterior-posterior plane. Once rotated in this manner,the dilator halves 48 a, 48 b are again separated to distract tissue ina generally anterior-posterior fashion.

As shown in FIG. 10, the posterior retractor blade 12 is thereafteradvanced along the posterior half 48 a of the split dilator 48. At thispoint, the posterior shim element 22 (FIGS. 4-5) may be advanced alongthe posterior retractor blade 12 (FIGS. 2-3) such that the shimextension 80 (distal end) is positioned in the posterior region of thedisc space as shown in FIG. 11 (with posterior half 48 a of the splitdilator 48 removed). The anterior retractor blade 14 may thereafter beadvanced along the anterior half 48 b of the split dilator 48 as shownin FIG. 12. At this point, secondary distraction may be undertakenaccording to the present invention by removing the anterior half 48 b ofthe split dilator 48 and introducing a plurality of sequentiallydilating cannula 50 between the posterior retractor blade 12 and theanterior retractor blade 14 as shown in FIG. 14. This serves to move theanterior retractor blade 14 anteriorly from the posterior retractorblade 12.

The retraction of the present invention is performed by expanding,modifying, and/or maintaining the distraction corridor to establishand/or maintain an operative corridor to the surgical target site. Asshown in FIG. 15, according to one embodiment, this is accomplished byintroducing the anterior shim element 24 along the anterior refractorblade 14 such that the shim extension 80 (distal region thereof) extendsinto the anterior region of the disc space. Supplemental retractorblades 16-18 (FIG. 6) may also optionally be introduced to define thecephalad and caudal sides of the operative corridor 15 as shown in FIGS.14-15. Once positioned as such, the retractor blades 12, 14 andsupplemental retractor blades 16, 18 may be locked in a positionrelative to the mounting structure 20 by tightening the respective nutsof the locking assemblies 36.

The retraction assembly 10 of the present invention, and in particularthe shim extensions 80 of the posterior and anterior shim elements 22,24 serve to prevent the ingress of unwanted or sensitive biologicalstructures (e.g., nerve roots and/or vasculature) into the surgicaltarget site, as well as prevent instruments from passing outside thesurgical target site and contacting surrounding tissues or structures.Once established, any of a variety of surgical instruments, devices, orimplants may be passed through and/or manipulated within the operativecorridor 15 depending upon the given surgical procedure.

According to yet another aspect of the present invention, any number ofdistraction components and/or retraction components (including but notlimited to those described herein) may be equipped to detect thepresence of (and optionally the distance and/or direction to) neuralstructures during the steps tissue distraction and/or retraction. Thisis accomplished by employing the following steps: (1) one or morestimulation electrodes are provided on the various distraction and/orretraction components; (2) a stimulation source (e.g. voltage orcurrent) is coupled to the stimulation electrodes; (3) a stimulationsignal is emitted from the stimulation electrodes as the variouscomponents are advanced towards or maintained at or near the surgicaltarget site; and (4) the patient is monitored to determine if thestimulation signal causes muscles associated with nerves or neuralstructures within the tissue to innervate. If the nerves innervate, thismay indicate that neural structures may be in close proximity to thedistraction and/or retraction components.

Neural monitoring may be accomplished via any number of suitablefashions, including but not limited to observing visual twitches inmuscle groups associated with the neural structures likely to found inthe tissue, as well as any number of monitoring systems, including butnot limited to any commercially available “traditional” electromyography(EMG) system (that is, typically operated by a neurophysiologist. Suchmonitoring may also be carried out via the surgeon-driven EMG monitoringsystem shown and described in the following commonly owned andco-pending “NeuroVision Applications” incorporated by reference intothis disclosure above. In any case (visual monitoring, traditional EMGand/or surgeon-driven EMG monitoring), the access system of the presentinvention may advantageously be used to traverse tissue that wouldordinarily be deemed unsafe or undesirable, thereby broadening thenumber of manners in which a given surgical target site may be accessed.

FIGS. 16-17 illustrate, by way of example only, a monitoring system 120of the type disclosed in the NeuroVision Applications suitable for usewith the surgical access system of the present invention. The monitoringsystem 120 includes a control unit 122, a patient module 124, and an EMGharness 126 and return electrode 128 coupled to the patient module 124,and a cable 132 for establishing electrical communication between thepatient module 124 and the surgical access system of the presentinvention. More specifically, this electrical communication can beachieved by providing, by way of example only, a hand-held stimulationcontroller 152 capable of selectively providing a stimulation signal(due to the operation of manually operated buttons on the hand-heldstimulation controller 152) to one or more connectors 156 a, 156 b, 156c. The connectors 156 a, 156 b, 156 c are suitable to establishelectrical communication between the hand-held stimulation controller152 and (by way of example only) the stimulation electrodes on theK-wire 42, the dilating cannula 44, the split-blade dilator 48, theretractor blades 12-18, and/or the shim elements 22, 24 (collectively“Surgical Access Instruments”).

In order to use the monitoring system 120, then, these Surgical AccessInstruments must be connected to the connectors 156 a, 156 b and/or 156c, at which point the user may selectively initiate a stimulation signal(preferably, a current signal) from the control unit 122 to a particularSurgical Access Instruments. Stimulating the electrode(s) on theseSurgical Access Instruments before, during and/or after establishingoperative corridor will cause nerves that come into close or relativeproximity to the Surgical Access Instruments to depolarize, producing aresponse in a myotome associated with the innervated nerve.

The control unit 122 includes a touch screen display 140 and a base 142,which collectively contain the essential processing capabilities(software and/or hardware) for controlling the monitoring system 120.The control unit 122 may include an audio unit 118 that emits soundsaccording to a location of a Surgical Access Instrument with respect toa nerve. The patient module 124 is connected to the control unit 122 viaa data cable 144, which establishes the electrical connections andcommunications (digital and/or analog) between the control unit 122 andpatient module 124. The main functions of the control unit 122 includereceiving user commands via the touch screen display 140, activatingstimulation electrodes on the surgical access instruments, processingsignal data according to defined algorithms, displaying receivedparameters and processed data, and monitoring system status and reportfault conditions. The touch screen display 140 is preferably equippedwith a graphical user interface (GUI) capable of communicatinginformation to the user and receiving instructions from the user. Thedisplay 140 and/or base 142 may contain patient module interfacecircuitry (hardware and/or software) that commands the stimulationsources, receives digitized signals and other information from thepatient module 124, processes the EMG responses to extractcharacteristic information for each muscle group, and displays theprocessed data to the operator via the display 140.

In one embodiment, the monitoring system 120 is capable of determiningnerve direction relative to one or more of the K-wire 42, dilatingcannula 44, split-retractor 48, retractor blades 12-18, and/or the shimelements 22, 24 before, during and/or following the creation of anoperative corridor to a surgical target site. Monitoring system 120accomplishes this by having the control unit 122 and patient module 124cooperate to send electrical stimulation signals to one or more of thestimulation electrodes provided on these Surgical Access Instruments.Depending upon the location within a patient (and more particularly, toany neural structures), the stimulation signals may cause nervesadjacent to or in the general proximity of the Surgical AccessInstruments to depolarize. This causes muscle groups to innervate andgenerate EMG responses, which can be sensed via the EMG harness 126. Thenerve direction feature of the system 120 is based on assessing theevoked response of the various muscle myotomes monitored by the system120 via the EMG harness 126.

By monitoring the myotomes associated with the nerves (via the EMGharness 126 and recording electrode 127) and assessing the resulting EMGresponses (via the control unit 122), the surgical access system of thepresent invention is capable of detecting the presence of (andoptionally the distant and/or direction to) such nerves. This providesthe ability to actively negotiate around or past such nerves to safelyand reproducibly form the operative corridor to a particular surgicaltarget site, as well as monitor to ensure that no neural structuresmigrate into contact with the retraction assembly 10 after the operativecorridor has been established. In spinal surgery, for example, this isparticularly advantageous in that the surgical access system of thepresent invention may be particularly suited for establishing anoperative corridor to an intervertebral target site in apostero-lateral, trans-psoas fashion so as to avoid the bony posteriorelements of the spinal column.

FIGS. 18-19 are exemplary screen displays (to be shown on the display140) illustrating one embodiment of the nerve direction feature of themonitoring system shown and described with reference to FIGS. 16-17.These screen displays are intended to communicate a variety ofinformation to the surgeon in an easy-to-interpret fashion. Thisinformation may include, but is not necessarily limited to, a display ofthe function 180 (in this case “DIRECTION”), a graphical representationof a patient 181, the myotome levels being monitored 182, the nerve orgroup associated with a displayed myotome 183, the name of theinstrument being used 184 (e.g. dilating cannula 44), the size of theinstrument being used 185, the stimulation threshold current 186, agraphical representation of the instrument being used 187 (in this case,a cross-sectional view of a dilating cannula 44) to provide a referencepoint from which to illustrate relative direction of the instrument tothe nerve, the stimulation current being applied to the stimulationelectrodes 188, instructions for the user 189 (in this case, “ADVANCE”and/or “HOLD”), and (in FIG. 19) an arrow 190 indicating the directionfrom the instrument to a nerve. This information may be communicated inany number of suitable fashions, including but not limited to the use ofvisual indicia (such as alpha-numeric characters, light-emittingelements, and/or graphics) and audio communications (such as a speakerelement). Although shown with specific reference to a dilating cannula(such as at 184), it is to be readily appreciated that the presentinvention is deemed to include providing similar information on thedisplay 140 during the use of any or all of the various Surgical AccessInstruments of the present invention, including the initial distractionassembly 40 (i.e. the K-wire 42, dilating cannula 44, and split dilator48), the secondary distraction assembly 50 (FIGS. 13-14), and/or theretractor blades 12-18 and/or shim elements 22, 24 of the retractionassembly 10.

The initial distraction assembly 40 (FIG. 7) may be provided with one ormore electrodes for use in providing the neural monitoring capabilitiesof the present invention. By way of example only, the K-wire 42 may beequipped with a distal electrode 60. This may be accomplished byconstructing the K-wire 42 for a conductive material, providing outerlayer of insulation extending along the entire length with the exceptionof an exposure that defines the electrode 60. The electrode 60 has anangled configuration relative to the rest of the K-wire 42 (such as, byway of example only, in the range of between 15 and 75 degrees from thelongitudinal axis of the K-wire 42). The angled nature of the electrode60 is advantageous in that it aids in piercing tissue as the K-wire 42is advanced towards the surgical target site.

The angled nature of the distal electrode 60 is also important in thatit provides the ability to determine the location of nerves or neuralstructures relative to the K-wire 42 as it is advanced towards orresting at or near the surgical target site. This “directional”capability is achieved by the fact that the angled nature of theelectrode 60 causes the electrical stimulation to be projected away fromthe distal portion of the K-wire 42 in a focused, or directed fashion.The end result is that nerves or neural structures which are generallycloser to the side of the K-wire 42 on which the electrode 60 isdisposed will have a higher likelihood of firing or being innervatedthat nerves or neural structures on the opposite side as the electrode60.

The direction to such nerves or neural structures may thus be determinedby physically rotating the K-wire 42 at a particular point within thepatient's tissue and monitoring to see if any neural stimulation occursat a given point within the rotation. Such monitoring can be performedvia visual observation, a traditional EMG monitoring, as well as thenerve surveillance system disclosed in the above-referenced NeuroVisionApplications. If the signals appear more profound or significant at agiven point within the rotation, the surgeon will be able tell where thecorresponding nerves or neural structures are, by way of example only,by looking at reference information (such as the indicia) on the exposedpart of the K-wire 42 (which reference point is preferably set forth inthe exact same orientation as the electrode 60).

The dilating cannula 44 and split dilator 48 may also be provided withelectrodes (flat electrodes 64 and angled electrodes 62, respectively)for the purpose of determining the location of nerves or neuralstructures relative to the dilating cannula 44 and split-dilator 48 areadvanced over the K-wire 44 towards or positioned at or near thesurgical target site. Electrodes 62, 64 may be provided via any numberof suitable methods, including but not limited to providing electricallyconductive elements within the walls of the dilating cannula 44 andsplit dilator 48, such as by manufacturing them from plastic or similarmaterial capable of injection molding or manufacturing them fromaluminum (or similar metallic substance) and providing outer insulationlayer with exposed regions (such as by anodizing the exterior of thealuminum dilator).

The secondary distraction assembly (including the sequential dilationassembly 50 of FIGS. 13-14) may be provided with one or more electrodesfor use in providing the neural monitoring capabilities of the presentinvention. By way of example only, it may be advantageous to provide oneor more electrodes on the dilating cannulae comprising the sequentialdilation assembly 50 for the purpose of conducting neural monitoringbefore, during and/or after the secondary distraction.

The retractor blades 12-18 and the shim elements 22, 24 of the presentinvention may also be provided with one or more electrodes for use inproviding the neural monitoring capabilities of the present invention.By way of example only, it may be advantageous to provide one or moreelectrodes on these components (preferably on the side facing away fromthe surgical target site) for the purpose of conducting neuralmonitoring before, during and/or after the refractor blades 12-18 and/orshim elements 22, 24 have been positioned at or near the surgical targetsite.

The surgical access system of the present invention may be sold ordistributed to end users in any number of suitable kits or packages(sterile and/or non-sterile) containing some or all of the variouscomponents described herein. For example, the retraction assembly 10 maybe provided such that the mounting assembly 20 is reusable (e.g.,autoclavable), while the refractor blades 12-18 and/or shim elements 22,24 are disposable. In a further embodiment, an initial kit may includethese materials, including a variety of sets of retractor blades 12-18and/or shim elements 22, 24 (and extensions 80) having varying (or“incremental”) lengths to account for surgical target sites of varyinglocations within the patient, optionally color-coded to designate apredetermined length.

As evident from the above discussion and drawings, the present inventionaccomplishes the goal of providing a novel surgical access system andrelated methods which involve creating a distraction corridor to asurgical target site, thereafter retracting the distraction corridor toestablish and maintain an operative corridor to the surgical targetsite, and optionally detecting the existence of (and optionally thedistance and/or direction to) neural structures before, during and/orafter the formation of the distraction and/or operative corridors.

The steps of distraction followed by retraction are advantageous becausethey provide the ability to more easily position an operativecorridor-establishing device through tissue that is strong, thick orotherwise challenging to traverse in order to access a surgical targetsite. The various distraction systems of the present invention areadvantageous in that they provide an improved manner of atraumaticallyestablishing a distraction corridor prior to the use of the retractionsystems of the present invention. The various retractor systems of thepresent invention are advantageous in that they provide an operativecorridor having improved cross-sectional area and shape (includingcustomization thereof) relative to the prior art surgical accesssystems. Moreover, by optionally equipping the various distractionsystems and/or retraction systems with one or more electrodes, anoperative corridor may be established through (or near) any of a varietyof tissues having such neural structures which, if contacted orimpinged, may otherwise result in neural impairment for the patient.

The surgical access system of the present invention can be used in anyof a wide variety of surgical or medical applications, above and beyondthe spinal applications discussed herein. By way of example only, inspinal applications, any number of implants and/or instruments may beintroduced through the working cannula 50, including but not limited tospinal fusion constructs (such as allograft implants, ceramic implants,cages, mesh, etc.), fixation devices (such as pedicle and/or facetscrews and related tension bands or rod systems), and any number ofmotion-preserving devices (including but not limited to nucleusreplacement and/or total disc replacement systems).

While certain embodiments have been described, it will be appreciated bythose skilled in the art that variations may be accomplished in view ofthese teachings without deviating from the spirit or scope of thepresent application. For example, with regard to the monitoring system120, it may be implemented using any combination of computer programmingsoftware, firmware or hardware. As a preparatory act to practicing thesystem 120 or constructing an apparatus according to the application,the computer programming code (whether software or firmware) accordingto the application will typically be stored in one or more machinereadable storage mediums such as fixed (hard) drives, diskettes, opticaldisks, magnetic tape, semiconductor memories such as ROMs, PROMs, etc.,thereby making an article of manufacture in accordance with theapplication. The article of manufacture containing the computerprogramming code may be used by either executing the code directly fromthe storage device, by copying the code from the storage device intoanother storage device such as a hard disk, RAM, etc. or by transmittingthe code on a network for remote execution. As can be envisioned by oneof skill in the art, many different combinations of the above may beused and accordingly the present application is not limited by the scopeof the appended claims.

1. A system for accessing a spinal disc of a lumbar spine through anoperative corridor, comprising: an initial dilating cannula configuredto create a tissue distraction corridor to a lumbar spine, wherein saidinitial dilating cannula is deliverable to a spinal disc along alateral, trans-psoas path to the lumbar spine, the distal tip region ofthe initial dilating cannula including a stimulation electrode exposedalong a tapered outer surface that is angled relative to a longitudinalaxis of said initial dilating cannula such that the stimulationelectrode delivers electrical stimulation away from the distal tipregion for nerve monitoring when the initial dilating cannula ispositioned in the lateral, trans-psoas path, a secondary distractionassembly comprising a plurality of dilators of sequentially larger widthdeliverable to the spinal disc along the lateral, trans-psoas path; anda refraction assembly comprising a mounting structure and four refractorblades releasably attachable to said mounting structure, wherein saidretraction assembly is configured to maintain an operative corridoralong the lateral, trans-psoas path to the lumbar spine that is enlargedfrom the tissue distraction corridor along the lateral, trans-psoas pathto the lumbar spine, wherein, when the four retractor blades arepositioned along the lateral, trans-psoas path, a first of the fourretractor blades is a posterior-most retractor blade, a second of thefour retractor blades is an anterior-most retractor blade, a third ofthe four retractor blades is a caudal-most retractor blade, and a fourthof the four retractor blades is a cephalad-most retractor blade, whereinsaid posterior-most retractor blade is spaced apart from saidanterior-most retractor blade when said four retractor blades maintainthe operative corridor along the lateral, trans-psoas path to the lumbarspine, wherein the operative corridor is dimensioned so as to pass animplant through the operative corridor along the lateral, trans-psoaspath to the lumbar spine.
 2. The system of claim 1, wherein saidposterior-most retractor blade is movable towards and away from saidanterior-most retractor blade.
 3. The system of claim 1, wherein saidanterior-most retractor blade is movable towards and away from saidposterior-most retractor blade.
 4. The system of claim 1, wherein saidcaudal-most retractor blade is movable towards and away from saidcephalad-most retractor blade.
 5. The system of claim 1, wherein saidcephalad-most retractor blade is movable towards and away from saidcaudal-most retractor blade.
 6. The system of claim 1, wherein saidposterior-most retractor blade and said anterior-most retractor bladeare slidably movable in the posterior-anterior direction.
 7. The systemof claim 1, wherein said cephalad-most retractor blade and saidcaudal-most retractor blade are slidably movable in the cephalad-caudaldirection.
 8. The system of claim 1, wherein said caudal-most refractorblade is spaced apart from said cephalad-most retractor blade when saidfour retractor blades maintain the operative corridor along the lateral,trans-psoas path to the lumbar spine.
 9. The system of claim 1, whereineach of the plurality of dilators of the secondary distraction assemblycomprises a stimulation electrode that delivers electrical stimulationfor nerve monitoring.
 10. The system of claim 1, wherein each of thefour retractor blades comprises a stimulation electrode that deliverselectrical stimulation for nerve monitoring.
 11. The system of claim 1,wherein the angular orientation of said posterior-most retractor bladeand said anterior-most retractor blade is independently adjustablerelative to said mounting structure such that the distal ends of saidposterior-most refractor blade and said anterior-most retractor bladeare capable of being angled toward each other.
 12. The system of claim1, wherein the angular orientation of said posterior-most retractorblade and said anterior-most retractor blade is independently aadjustable relative to said mounting structure such that the distal endsof said posterior-most refractor blade and said anterior-most retractorblade are capable of being angled away from each other.
 13. The systemof claim 1, further comprising a monitoring system that delivers anelectrical stimulation signal to the stimulation electrode of theinitial dilating cannula, monitors electromyographic activity detectedby a set of sensor electrodes in muscle myotomes associated with nervesin the vicinity of the spinal disc, and displays, to a user, dataindicating a stimulation threshold required to obtain theelectromyographic activity in at least one of said muscle myotomes. 14.The system of claim 13, wherein the monitoring system comprises acontrol unit having a video display device, a patient module connectedto the control unit via a data cable, and an EMG sensor harness havingthe set of sensor electrodes connected to the patient module.
 15. Thesystem of claim 14, wherein the control unit receives signals from thepatient module and processes EMG response output from the sensorelectrodes to extract characteristic information for each of said musclemyotomes.
 16. The system of claim 14, wherein the initial dilatingcannula is connected to said monitoring system via a removable connectorthat establishes electrical communication between the monitoring systemand the stimulation electrode at the distal tip region of the initialdilating cannula.
 17. A system for accessing a spinal disc of a lumbarspine through an operative corridor, comprising: an initial dilatingcannula configured to create a tissue distraction corridor to a lumbarspine, wherein said initial dilating cannula is deliverable to a spinaldisc along a lateral, trans-psoas path to the lumbar spine, the distaltip region of the initial dilating cannula including a stimulationelectrode exposed along a tapered outer surface that is angled relativeto a longitudinal axis of said initial dilating cannula such that thestimulation electrode is configured to deliver electrical stimulationaway from the distal tip region for nerve monitoring when the initialdilating cannula is positioned in the lateral, trans-psoas path, asecondary distraction assembly comprising a plurality of dilators ofsequentially larger diameter deliverable to the spinal disc along thelateral, trans-psoas path; and a retraction assembly comprising a bladeholder apparatus and a plurality of retractor blades releasablyattachable to said blade holder apparatus, wherein said retractionassembly is configured to maintain an operative corridor along thelateral, trans-psoas path to the lumbar spine that is enlarged from thetissue distraction corridor along the lateral, trans-psoas path to thelumbar spine, wherein the retraction assembly further comprises afixation element that is releasably attachable to a first refractorblade of the plurality of retractor blades such that a distal portion ofsaid fixation element extends distally of a distal end of the said firstretractor blade and is configured to penetrate into the lumbar spine foraffixing the first retractor blade to the lumbar spine, said fixationelement including a proximal portion having a width that is greater thana maximum width of said distal portion of said fixation element, whereina second retractor blade of the plurality of retractor blades is movableaway from the first retractor blade for establishing said operativecorridor to the lumbar spine between said first and second retractorblades, wherein the operative corridor is dimensioned so as to pass animplant through the operative corridor along the lateral, trans-psoaspath to the lumbar spine.
 18. The system of claim 17, wherein saidfixation element of the retraction assembly is slidably engageable withsaid first retractor blade.
 19. The system of claim 18, wherein saiddistal portion of said fixation element is configured to penetrate intothe lumbar spine so as to affix said first retractor blade in said fixedrelative position.
 20. The system of claim 17, further comprising amonitoring system that delivers an electrical stimulation signal to thestimulation electrode of the initial dilating cannula, monitorselectromyographic activity detected by a set of sensor electrodes inmuscle myotomes associated with nerves in the vicinity of the spinaldisc, and displays numeric data indicating a stimulation thresholdrequired to obtain the electromyographic activity in at least one ofsaid muscle myotomes.
 21. The system of claim 20, wherein the monitoringsystem comprises a control unit having a video display device, a patientmodule connected to the control unit via a data cable, and an EMG sensorharness having the set of sensor electrodes connected to the patientmodule.
 22. The system of claim 21, wherein the control unit receivessignals from the patient module and processes EMG response output fromthe sensor electrodes to extract characteristic information for each ofsaid muscle myotomes.
 23. The system of claim 21, wherein the initialdilating cannula is connected to said monitoring system via a removableconnector that establishes electrical communication between themonitoring system and the stimulation electrode at the distal tip regionof the initial dilating cannula.
 24. The system of claim 20, wherein themonitoring system displays said numeric data comprises a value ofmilliAmps indicative of the stimulation threshold required to obtain theelectromyographic activity.
 25. The system of claim 17, wherein at leastone of the plurality of dilators of the secondary distraction assemblycomprises a stimulation electrode that delivers electrical stimulationfor nerve monitoring.
 26. The system of claim 17, wherein each of theplurality of retractor blades comprises a stimulation electrode thatdelivers electrical stimulation for nerve monitoring.
 27. The system ofclaim 17, wherein an angular orientation of said first retractor bladeand said second retractor blade is independently adjustable relative tosaid blade holder apparatus.
 28. The system of claim 27, wherein theangular orientation of said first retractor blade and said secondretractor blade is independently adjustable such that the distal ends ofsaid first retractor blade and said second retractor blade are capableof being angled toward each other.
 29. The system of claim 27, whereinthe angular orientation of said first retractor blade and said secondretractor blade is independently adjustable such that the distal ends ofsaid first retractor blade and said second retractor blade are capableof being angled away from each other.
 30. The system of claim 17,wherein said first retractor blade is configured to be delivered alongthe lateral, trans-psoas path before said second retractor blade isdelivered along the lateral, trans-psoas path.
 31. The system of claim17, wherein said second retractor blade is movable away from said firstrefractor blade under force from said plurality of dilators ofsequentially larger diameter.
 32. The system of claim 17, wherein saidplurality of retractor blades comprises four retractor blades.
 33. Thesystem of claim 32, wherein when the four retractor blades are deliveredalong the lateral, trans-psoas path, the first retractor blade of thefour retractor blades is a posterior-most retractor blade, the secondretractor blade of the four retractor blades is an anterior-mostretractor blade, a third retractor blade of the four retractor blades isa caudal-most retractor blade, and a fourth retractor blade of the fourrefractor blades is a cephalad-most retractor blade.